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The chemical nature of the carbon precursor in bias-enhanced nucleation of CVD diamond
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DIAMOND RELATED h .TERiALS AND
ELSEVIER
Diamond and Related Materials 6 (1997) 526-531
The chemical nature of the carbon precursor in bias-enhanced nucleation of CVD diamond I. Gouzman a.b, B. Fisgeer a,b, y . Avigal a, R. Kalish a,c, A. Hoffman a,b a Solid State btstitute, Technion - Israel btstitute of Technology, Haifa 32000, Israel b Chemistry Department, Technion - Israel hzstitute of Technology, Haifa 32000, Israel c Physics Department, Technion - Israel bzstitute of Tedmology, Haifa 32000, Israel
Abstract The effect of sample biasing on CVD diamond nucleation on Si(100) substrates by the MW method has been previously addressed; however, the question as to the changes a silicon substrate undergoes under bias treatment is still open. In the present work we address this question by investigating the chemical reactivity of the material deposited during substrate bias. The different etching behavior in a hydrogen plasma of the possible precursor layer formed on silicon during the bias treatment is utilized.
Silicon substrates were subjected to a three-step sequential process: (i) bias pretreatment, (ii) exposure to a pure hydrogen plasma and (iii) deposition under normal conditions. The characterization of the deposited films following each of these stages was carried out by Raman, AES and AFM. It was found that the carbon precursor to diamond growth, formed during the biasing stage, is stable under the etching process. This result suggests that the deposited material at this stage does not consist solely of a carbon network etchable by the hydrogen plasma, such as amorphous carbon (sp2 o r s p 3 bonded ) and microcrystalline graphite or silicon carbide. It may therefore, be suggested that the precursor to diamond growth deposited during the bias process consists of non-etchable nanocrystailite diamond particles. 43 1997 Elsevier Science S.A. Ke.vword~v Bias-enhanced nucleation; Diamond
1. Introduction Over the last few years the bias enhanced nucleation (BEN) method has become a widely used technique for achieving nucleation during the first stage of CVD diamond growth. In this method the enhancement of nucleation is caused by in situ pretreatment of the substrate surface (usually Si) in a MW hydrogen plasma with a high methane concentration (from 5% to 40%) and the application of d.c. bias ( < - 7 0 V ) to the substrate [!-5]. After the predeposition process, diamond growth is carried out under normal CVD conditions. A similar bias pretreatment was used to enhance nucleation on //-SIC [6], TiC [7], Ti and Hf [8]. At present, the bias nucleation yields the highest nucleation densities known. In addition, heteroepitaxial diamond films have been obtained on Si and //-SIC following BEN [9-15]. In spite of the wide application of this technique, the mechanism responsible for the enhancement of diamond nucleation caused by the bias pretreatment is unclear. A number of ideas have been proposed to explain the 0925-9635/97/$ !7.00 © 1997 Elsevier Science S.A. All rights reserved. PH S0925-9635 ( 96 ) 00702-9
effect of BEN based on the experimental data, among them: the ion subplantation model [16]; gas phase composition changes [17]; increased surface mobilities of adsorbed species [2,10]; and others. An important question which has been examined by many authors is the chemical nature of the carbon precursor formed during sample biasing [2, 5]. The main possibilities considered are: (i) the formation of amorphous carbon layer [18] or polymer-like hydrocarbon structure [19], (ii) sp 3 clusters [1,20], (iii) sp2-bonded graphitic carbon [5,21] and (iv) high density nanocrystalline diamond nuclei in a nondiamond carbon matrix [22-24]. In the pcesent work, in addition to spectroscopic and microscopic examination, we use hydrogen plasma exposure as a chemical probe to determine the nature of carbon precursor for diamond deposition due to BEN. This method is based on the difference in the etching rate of various carbon phases in the hydrogen plasma. Graphitic and amorphous, sp 2- as well as sp3-bonded carbon structures undergo a very rapid etching process, whereas the etching rate of diamond is
I. Gouzman et al. / Diamond and Related Materials 6 (1997) 526-531
negligible [25,26]. The etching rate of HOPG was measured in the present work to be ~ 6 lam h-~. An important experimental parameter to be studied is the minimum biasing time needed for nucleation enhancement. It was found that the bias pretreatment time affects both the nucleation density and the quality of the diamond film. Though many researchers have applied a biasing time of about 15 min [ 1,2,21 ], we observed a significant nucleation enhancement even after 1.5 min of biasing and found that longer biasing times result in a deterioration of the overgrown diamond film.
527
position of the surface, carbon hybridization and silicon bonding. Micro-Raman measurements of the deposited material were performed using the 514.5 nm line of an Ar laser (Dilor) in a backscattering geometry. Laser powers of 5-8 mW and magnifications of 100 were used. The laser spot size was about 2 lam. From the Raman spectroscopy the presence of different carbon phases could be determined.
3. Results and discussion
3.1. Tire#~uence of bias pretreatment time 2. Experimental
2.1. Filmpreparation Diamond growth was carried out in a tubular MW (2.45 GHz) plasma CVD apparatus. Substrate biasing was performed using a spiral tungsten electrode brought in contact with the plasma from the top of the quartz tube. The deposition was carried out on p-type Si(100) substrates of 1 x 1 cm 2 size and 0.6 mm thickness. The substrates were chemically cleaned with acetone in an ultrasonic bath for 10 min prior to mounting in the reactor. The substrate temperature was controlled by the MW power density and the location of the silicon substrate with respect to the plasma ball. It was measured by an infrared pyrometer through a viewing port normal to the substrate. After pumping the chamber, the substrates were etched in situ in a hydrogen plasma for 8 min to clean the surface and remove the native surface oxide layer. This step was followed by the bias pretreatment for various times in 6% CH4/H2 plasma at a bias voltage of - 2 0 0 V. The bias voltage was then turned off and the standard deposition process was continued for 1 h with a reduced methane concentration of 0.5%. Deposition parameters were: total flow rate 100 sccm; total gas pressure 45 Torr; substrate temperature 800°C; microwave power 400W. Hydrogen plasma exposure was performed after bias pretreatment, prior to standard deposition.
2.2. Characterizationof the depositedfihns Characterization of the deposited films was carried out by Raman, Auger electron spectroscopy (AES), and atomic force microscopy (AFM). AFM measurements were carried out in a Topometrics TMX 2010 instrument, operating in non-contact (attractive)mode under ambient conditions. Nucleation density and the topography of the deposited films could be determined from the AFM pictures. AES analyses were performed using a four-grid retarding field analyzer and primary electrons with an energy of 1 keV. From the AES measurements, information was obtained regarding the chemical com-
The nucleation density of deposited diamond crystallites was calculated using AFM pictures (data not shown), taken after 1 h of deposition, following the biasing stage. It was found that the nucleation density increases from 1 x 108 particles/cm2 after 1.5 min of bias pretreatment, to 8 x 108 particles/cm2 after 3 min of bias pretreatment. Then, 7 min of bias pretreatment followed by 1 h of deposition results in the formation of a continuous film, for which the nucleation density cannot be calculated precisely. These results may be compared with the kinetic data observed by Gerber et al. [21], who found that the nucleation density remains low for biasing time up to 5 min and then increases exponentially with biasing time. This comparison shows that nucleation density may be a very sensitive function of particular bias and deposition conditions. Raman spectra of samples after I h of deposition under normal conditions are shown in Fig. ! for different bias pretreatment times. All Raman spectra display the characteristic diamond line at 1333 cm- 1. In addition, the Raman spectrum taken after l h of deposition, following 7 min of bias pretreatment, reveals the presence of a broad band centered at ~1540 c m - t , which is characteristic of amorphous carbon. The intensity of this band in the spectrum, taken after 1 h of deposition following 3 mJn of bias pretreatment, is much lower. Thus, it seems that the prolonged biasing time results in the formation of some type of amorphous carbon phase, whereas a short biasing time (<3 rain) leads to the deposition of good quality diamond films in our particular MW-CVD system.
3.2. Hydrogenplasma etch#lg of the deposit produced by sample biasing The chemical nature of the carbon precursor to CVD diamond formed during the biasing stage was studied through the effect of in situ hydrogen plasma exposure prior to growth. In these experiments, the Si(100) substrates were put through a three-step process: (i) bias pretreatment, (ii) exposure to a pure hydrogen plasma and (iii) deposition under normal conditions.
528
L Gou:man et al. / Diamond and Related Materials 6 (1997) 526-531
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One can expect that if the carbon precursor, formed during the biasing stage, undergoes chemical etching in a hydrogen plasma atmosphere, then no diamond film should be formed at the end of the three-step deposition process. However, our AFM studies did not detect any difference in nucleation density as a result of exposure of the substrate to a pure hydrogen plasma. This effect was observed alter 7 min of bias pretreatment followed by 30min of hydrogen plasma exposure and l h of deposition. Fig. 2-I shows the Raman spectra of CVD films grown for I h, after 7 rain of bias pretreatment and followed by exposure to hydrogen plasma for different times, ranging from 0 to 30 min. A relatively prolonged bias time was chosen in order to obtain sufficient intensity of spectroscopic signals (Raman and AES) at the end of the biasing process. No visible effect of hydrogen plasma exposure is seen in the Raman spectra. The characteristic diamond peak at 1333 cm-1 was observed in all spectra, even after 30 min of hydrogen plasma exposure prior to film deposition. In Fig. 2-11 (b-eL high resolution C(KVV) Auger line shapes are presented as a function of the exposure time. The Auger spectrum taken for natural diamond is also shown in Fig. 2-11(a), as a comparison. The position of the C( KVV ) Auger line, and the line shape measured for the deposited films, are very similar to those of natural diamond, particularly after 20 and 30 min of hydrogen plasma exposure (Fig. 2-II (d) and (e)). It is
Fig. 2. I. Raman spectra of the different samples after 1 h of deposition under normal conditions following exposure of the films to hydrogen plasma for different times. II. High resolution C(KVV) electron excited Auger spectra taken after 1 h of deposition under normal conditions following exposure of the films to hydrogen plasma for different times.
important to stress that the Raman spectra of these films were also rather similar and displayed a significant contribution by an amorphous carbon phase. These results reflect the depth non-uniformity of the deposited films due to the different sampling depth of Raman and Auger spectroscopy. AES is a surface-sensitive technique, whereas Raman spectroscopy analyzes the bulk of the film. In order to gain insight into the nature of the carbon phase formed during sample biasing, films which underwent stages (i) and (ii) mentioned above (i.e., bias treatment and exposure to hydrogen plasma, without consequent CVD growth) were analyzed by Raman, AES and AFM. Fig. 3 shows the AFM pictures of Si substrates after different surface treatments: (a) untreated, (b) after 30 min of hydrogen plasma exposure, (c) after 7 min of the bias treatment and (d) after 7 min of the bias treatment followed by 30 min of hydrogen plasma
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Fig. 3. AFM pictures of Si substrate after diffeient surface treatments: untreated (a), after 30 min of hydrogen plasma exposure (b), after 7 min of the bias treatment (c) and after 7 min of the bias treatment followed by 30 min of hydrogen plasma exposure (d).
I. Gouzman et al. / Diamond and Related Materials 6 (1997) 526-531
exposure. Hydrogen plasma exposure, applied to virgin Si, resulted in an increase of the surface roughness (Fig. 3(b)). We attempted to grow diamond film on the substrate pretreated in the hydrogen plasma; however, no diamond particles were detected after 1 h of deposition (data not shown). Hence, the nucleation enhancement observed after 7 min of bias pretreatment and 30 min of hydrogen plasma exposure, followed by 1 h of CVD growth, is really a result of the stability under the etching conditions of the carbon precursor for diamond nucleation formed during the biasing stage. It was found that 7 min of bias treatment resulted in the appearance of large "hillocks" on the substrate surface (Fig. 3(c)). An increase in surface irregularity, concurrent with a decrease in the average height of the surface image, was observed after 30 min of hydrogen plasma exposure following 7 min of bias treatment (Fig. 3(d)). These changes may be associated with the roughening of the substrate surface by hydrogen plasma erosion and the partial etching of carbon and silicon atoms. In Fig. 4 the Raman spectra taken after 7 min of biasing pretreatment are shown as a function of hydrogen plasma exposure time. From these data it may be concluded that: (a) All spectra show the presence of two broad bands with apparent maxima at ~ 1333 and 1583 cm-1. These bands may be considered as D and G peaks of micro(or nano-) crystalline graphite, respectively. The large shift of the D peak (the D peak for microcrystalline
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529
graphite is expected to be centered at 1350 cm -1) is possibly due to crystalline size effects. It may be suggested that very small diamond particles, formed during the biasing stage, contribute to the intensity of the Raman band at about 1333 cm-1. (b) The intensity of these lines does not change significantly after 10 or 20 min of hydrogen plasma exposure (Figs. 4(b) and (c)). This result, along with AFM studies, indicates a surprising stability of the carbon precursor deposited during sample biasing under exposure to hydrogen plasma. Hence, it is suggested that this precursor consists of nanocrystalline diamond particles, possibly embedded in a dense matrix of nondiamond carbon. This suggestion is supported by the results observed using selected area electron diffraction and X-ray diffraction, as well as EELS, in the works of McGinnis et al. [23] and Reinke et al. [24], respectively. Also, TEM examination of diamond films deposited using MW-BEN to promote nucleation revealed the presence of amorphous carbon at the silicon-thin film interface [2]. (c) An abrupt decrease in the intensity of the Raman lines was found in the spectra taken after 30 min of exposure to hydrogen plasma. However, a Raman spectrum taken after 1 h of deposition, following 7 min of sample bias and 30 min of hydrogen plasma exposure, does not differ significantly from those taken after shorter exposure times (see Fig. 2-l). The abrupt decrease in the Raman band intensity between 20 and 30 min of hydrogen plasma exposure may be associated with the relationship between the sampling depth of the Raman technique and the thickness of the deposited carbon layer. Thus, one can conclude that a very thin, perhaps non-continuous film of carbon precursor which remains on the Si substrate surlhce after exposure to hydrogen plasma for 30 min, is sufficient tbr nucleation enhancement. The surface composition of the substrates which underwent bias pretreatment followed by hydrogen plasma exposure (without the deposition stage), was assessed by AES. The Auger spectra of these samples show the presence of Si(LVV ), O(KLL) and C(KVV) Auger lines. High resolution C(KVV) and Si(LVV) Auger line shapes of substrates alter 7 rain of bias pretreatment, are presented as a function of hydrogen plasma exposure time in Fig. 5-1 and Fig. 5-I1, respectively. As a comparison, the C(KVV) and Si(LVV) Auger line shapes, measured for a reference silicon carbide surface, are shown in Fig. 5-1(a) and Fig. 5ll(a), respectively. The C(KVV) Auger line, measured for these samples, was observed at 270.5 eV. The line shapes are not characteristic of that measured for diamond. At the same time, the Si(LVV) Auger line shape, measured after 7 rain of the bias pretreatment, indicates the presence of silicon carbide on the substrate surface, as determined by the characteristic Si(LVV) line at
I. Gouzman et ai. / Diamoml ami Rehtted Materials 6 (1997) 526-531
530
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crystalline graphite, or silicon carbide. Therefore, it may be suggested that the precursor to diamond growth, deposited during the biasing process, consists of nanocrystalline diamond particles. (2) Under our experimental MW-BEN and subsequent deposition conditions it has been observed that a very short bias pretreatment time (1.5 min) is sufficient for nucleation of a high quality diamond film with a relatively high nucleation density (1 x 108 particles/cm2).
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Fig. 5. 1. High resolution C(KVV) electron excited Auger spectra taken alter 7 rain of bias following the exposure of the films to hydrogen plasma for different times. II. High resolution Si(LVV) electron excited Auger spectra taken after 7 rain of bias following exposure of the films to hydrogen plasma for different times.
87 eV (Fig. 5-11(b)). The intensity of this line decreases significantly after l0 min of hydrogen plasma exposure, concurrent with the appearance of features characteristic of silicon oxide at 62 and 78 eV [27] (Fig. 5-11(c)). After longer exposure times (20 and 30 min) no silicon carbide was detected on the substrate surface. From the observed results one can conclude that: (a) The C(KVV) Auger lines, measured after 0 and 10 min of exposure in hydrogen plasma (Fig. 5-1(b) and Fig. 5-1(c)), are composed of Auger electrons, originating from (i) carbon a~oms bonded to silicon in a silicon carbide configuration and (ii) carbon atoms bonded to each other. Unfortunately, it is difficult to assess the chemical configuration of these carbon atoms from the complex C(KVV) Auger line shape. (b) Only carbon atoms bonded to each other contribute to the C(KVV) Auger line, measured after 20 and 30 rain of hydrogen plasma exposure (Fig. 5-l(d) and Fig. 5-1(e)). It seems that these carbon atoms are those that serve as nucleation sites for subsequent diamcnd deposition and not SiC as the latter is etched by atomic hydrogen without a degradation effect on diamond growth. This result is strongly supported by the recent publication of Kulisch et al. [19].
4. Conclusions (1) We propose that hydrogen plasma exposure may be used as a chemical probe to determine the nature of the carbon precursor for diamond deposition, formed during the biasing process. In our initial studies it was found that this carbon phase was stable under the etching process, suggesting that it does not consist solely of a carbon network etchable by the hydrogen plasma, such as amorphous carbon (sp 2- or spa-bonded), micro-
Acknowledgement The authors gratefully acknowledge financial support of the Israeli Ministry of Science and Arts, project No. 1000 004 and the Technion Foundation for Research.
References [1] S. Yugo, T. Kanai, T. Kimura and T. Muto, Appl. Phys. Lett., 59 ( 1991 ) 417. [2] B.R. Stoner, G.-H.M. Ma, S.D. Wolter and J.T. Glass, Phys. Rev. B, 45 (1992) 11067. [3] X. Jiang, R. Six, C.P. Klages, R. Zachai, M. Hartweg and H.-J. Ftisser, Diamond Relat. Mater.. 2 (1992) 407. [4] S. Yugo, T. Kimura, H. Nanai, Diamond Relat. Mater.. 2 ( 1992 ) 407. [51 J. Gerber, S. Sattel, K. Jung, H. Ehrhardt and J. Robertson, Dhmumd Rehtt. Mater.. 4 (1995) 559. [6] B.R. Stonel' and ,I.T. Glass, Appl. Phys. Lett., 6(1 11992) 698. [7] S.D. Wolter, M.T. McClure, J.T. Glass and B.R. Stoner, Appl. Ph),s Lett.. 66 (1995) 2810. [8] S.D. Wolter, J.T. Glass and B.R. Stoner, J. AppL Phys., 77 (1995) 5119. [9] X. Jiang, C.P. Klages, R. Zachai, M. Hartweg and H.J. Fusser, Appl. Phys. Left.. 62 (1993) 3438. [10] X. Jiang, K. Schiffmann, A. Westphal and C.P. Klages, Appl. Phys. [,,tt.. 63 ( 1993 ) 1203. [!1] R. Kohl, C, Wild, N. Herres, P. Koidl, B.R. Stoner and J.T. Glass, Appl. Plow. Lett., 63 (1993) 1792. [12] S.D. Wolter, B.K. Stoner, J.T. Glass, P.J. Ellis, D.S. Buhaenko, C.E. Jenkins and P. Southworth, Appl. Phys. Lett.. 62 (1993) 1215. [13] M. Schreck and B. Stritzker, Phys. Star. So/. (a), 154 (1996} 197. [14] M,G. Jubber and D.K. Milne, Phys. Star. Sol. (a). 154 (1996) 185. [15] X. Jiang and C, P. Klages, Phys. Stat. Sol. (a), 154 (1996) 175. [',6] J. Robertson, Diamond Relat. Mater.. 2 (1993) 984; 3 (1994) 361; 4 ( 1995 ) 549. [17] Y. Shigesato, R.E. Boekenhauer and B.W. Sheldom, Appl. Phys. Lett., 63 (1993) 314. [18] B.W. Sheldon, R. Csencsits, R. Rankin, Boekenhauer R.E. and Y. Shigesato, J. Appl. Phys., 75 (1994) 5001. [19] W. Kulisch, L. Ackermann and B. Sobisch, Phys. Star. Sol. (a), 154 (1996) 155. [20] S. Yugo, K. Semoto, K. Hoshina, T. Kimura and H. Nanai, Diamond Relat. Mater., 4 (1995) 903. [21] J. Gerber, S. Sattel, H. Ehrhardt, J. Robertson, P. Wutzinger and P. Pongratz, J. Appl. Ph)w.. 79 (1996) 4388.
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[22] R. Stockel, K. Janischowsky, S. Rohmfeld, J. Ristein, M Hundhausen and L. Ley, J. Appl. Phys., 79 (1996) 768. [23] S.P. McGinnis, M.A. Kelly, S.B. Hagstr6m and R.L. Alvis, J. Appl. Phys., 79 (1996) 170. [24] P. Reinke, P. Ka.nia, P. Oelhafen and R. Guggenheim, Appl. Phys. Lett., 68 (1996) 22.
5? 1
[25] Z. Feng, M.A. Brewer, K. Komvopoulos, I.G. Brown and D.B. Bogy, J. Mater. Res., 10 (1995) 165. [26] A. Hoffman, A. Fayer, A. Laikhtman and R. Brener, J. Appl. Phys., 77 (1995) 3126. [27] O.S. Chia, C.M. Younes, P.G. Partridge, G.C. Allen, P.W. May and C.A. Rego, J. Mater Sci., 29 (1994) 6397.
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DIAMOND RELATED h .TERiALS AND
ELSEVIER
Diamond and Related Materials 6 (1997) 526-531
The chemical nature of the carbon precursor in bias-enhanced nucleation of CVD diamond I. Gouzman a.b, B. Fisgeer a,b, y . Avigal a, R. Kalish a,c, A. Hoffman a,b a Solid State btstitute, Technion - Israel btstitute of Technology, Haifa 32000, Israel b Chemistry Department, Technion - Israel hzstitute of Technology, Haifa 32000, Israel c Physics Department, Technion - Israel bzstitute of Tedmology, Haifa 32000, Israel
Abstract The effect of sample biasing on CVD diamond nucleation on Si(100) substrates by the MW method has been previously addressed; however, the question as to the changes a silicon substrate undergoes under bias treatment is still open. In the present work we address this question by investigating the chemical reactivity of the material deposited during substrate bias. The different etching behavior in a hydrogen plasma of the possible precursor layer formed on silicon during the bias treatment is utilized.
Silicon substrates were subjected to a three-step sequential process: (i) bias pretreatment, (ii) exposure to a pure hydrogen plasma and (iii) deposition under normal conditions. The characterization of the deposited films following each of these stages was carried out by Raman, AES and AFM. It was found that the carbon precursor to diamond growth, formed during the biasing stage, is stable under the etching process. This result suggests that the deposited material at this stage does not consist solely of a carbon network etchable by the hydrogen plasma, such as amorphous carbon (sp2 o r s p 3 bonded ) and microcrystalline graphite or silicon carbide. It may therefore, be suggested that the precursor to diamond growth deposited during the bias process consists of non-etchable nanocrystailite diamond particles. 43 1997 Elsevier Science S.A. Ke.vword~v Bias-enhanced nucleation; Diamond
1. Introduction Over the last few years the bias enhanced nucleation (BEN) method has become a widely used technique for achieving nucleation during the first stage of CVD diamond growth. In this method the enhancement of nucleation is caused by in situ pretreatment of the substrate surface (usually Si) in a MW hydrogen plasma with a high methane concentration (from 5% to 40%) and the application of d.c. bias ( < - 7 0 V ) to the substrate [!-5]. After the predeposition process, diamond growth is carried out under normal CVD conditions. A similar bias pretreatment was used to enhance nucleation on //-SIC [6], TiC [7], Ti and Hf [8]. At present, the bias nucleation yields the highest nucleation densities known. In addition, heteroepitaxial diamond films have been obtained on Si and //-SIC following BEN [9-15]. In spite of the wide application of this technique, the mechanism responsible for the enhancement of diamond nucleation caused by the bias pretreatment is unclear. A number of ideas have been proposed to explain the 0925-9635/97/$ !7.00 © 1997 Elsevier Science S.A. All rights reserved. PH S0925-9635 ( 96 ) 00702-9
effect of BEN based on the experimental data, among them: the ion subplantation model [16]; gas phase composition changes [17]; increased surface mobilities of adsorbed species [2,10]; and others. An important question which has been examined by many authors is the chemical nature of the carbon precursor formed during sample biasing [2, 5]. The main possibilities considered are: (i) the formation of amorphous carbon layer [18] or polymer-like hydrocarbon structure [19], (ii) sp 3 clusters [1,20], (iii) sp2-bonded graphitic carbon [5,21] and (iv) high density nanocrystalline diamond nuclei in a nondiamond carbon matrix [22-24]. In the pcesent work, in addition to spectroscopic and microscopic examination, we use hydrogen plasma exposure as a chemical probe to determine the nature of carbon precursor for diamond deposition due to BEN. This method is based on the difference in the etching rate of various carbon phases in the hydrogen plasma. Graphitic and amorphous, sp 2- as well as sp3-bonded carbon structures undergo a very rapid etching process, whereas the etching rate of diamond is
I. Gouzman et al. / Diamond and Related Materials 6 (1997) 526-531
negligible [25,26]. The etching rate of HOPG was measured in the present work to be ~ 6 lam h-~. An important experimental parameter to be studied is the minimum biasing time needed for nucleation enhancement. It was found that the bias pretreatment time affects both the nucleation density and the quality of the diamond film. Though many researchers have applied a biasing time of about 15 min [ 1,2,21 ], we observed a significant nucleation enhancement even after 1.5 min of biasing and found that longer biasing times result in a deterioration of the overgrown diamond film.
527
position of the surface, carbon hybridization and silicon bonding. Micro-Raman measurements of the deposited material were performed using the 514.5 nm line of an Ar laser (Dilor) in a backscattering geometry. Laser powers of 5-8 mW and magnifications of 100 were used. The laser spot size was about 2 lam. From the Raman spectroscopy the presence of different carbon phases could be determined.
3. Results and discussion
3.1. Tire#~uence of bias pretreatment time 2. Experimental
2.1. Filmpreparation Diamond growth was carried out in a tubular MW (2.45 GHz) plasma CVD apparatus. Substrate biasing was performed using a spiral tungsten electrode brought in contact with the plasma from the top of the quartz tube. The deposition was carried out on p-type Si(100) substrates of 1 x 1 cm 2 size and 0.6 mm thickness. The substrates were chemically cleaned with acetone in an ultrasonic bath for 10 min prior to mounting in the reactor. The substrate temperature was controlled by the MW power density and the location of the silicon substrate with respect to the plasma ball. It was measured by an infrared pyrometer through a viewing port normal to the substrate. After pumping the chamber, the substrates were etched in situ in a hydrogen plasma for 8 min to clean the surface and remove the native surface oxide layer. This step was followed by the bias pretreatment for various times in 6% CH4/H2 plasma at a bias voltage of - 2 0 0 V. The bias voltage was then turned off and the standard deposition process was continued for 1 h with a reduced methane concentration of 0.5%. Deposition parameters were: total flow rate 100 sccm; total gas pressure 45 Torr; substrate temperature 800°C; microwave power 400W. Hydrogen plasma exposure was performed after bias pretreatment, prior to standard deposition.
2.2. Characterizationof the depositedfihns Characterization of the deposited films was carried out by Raman, Auger electron spectroscopy (AES), and atomic force microscopy (AFM). AFM measurements were carried out in a Topometrics TMX 2010 instrument, operating in non-contact (attractive)mode under ambient conditions. Nucleation density and the topography of the deposited films could be determined from the AFM pictures. AES analyses were performed using a four-grid retarding field analyzer and primary electrons with an energy of 1 keV. From the AES measurements, information was obtained regarding the chemical com-
The nucleation density of deposited diamond crystallites was calculated using AFM pictures (data not shown), taken after 1 h of deposition, following the biasing stage. It was found that the nucleation density increases from 1 x 108 particles/cm2 after 1.5 min of bias pretreatment, to 8 x 108 particles/cm2 after 3 min of bias pretreatment. Then, 7 min of bias pretreatment followed by 1 h of deposition results in the formation of a continuous film, for which the nucleation density cannot be calculated precisely. These results may be compared with the kinetic data observed by Gerber et al. [21], who found that the nucleation density remains low for biasing time up to 5 min and then increases exponentially with biasing time. This comparison shows that nucleation density may be a very sensitive function of particular bias and deposition conditions. Raman spectra of samples after I h of deposition under normal conditions are shown in Fig. ! for different bias pretreatment times. All Raman spectra display the characteristic diamond line at 1333 cm- 1. In addition, the Raman spectrum taken after l h of deposition, following 7 min of bias pretreatment, reveals the presence of a broad band centered at ~1540 c m - t , which is characteristic of amorphous carbon. The intensity of this band in the spectrum, taken after 1 h of deposition following 3 mJn of bias pretreatment, is much lower. Thus, it seems that the prolonged biasing time results in the formation of some type of amorphous carbon phase, whereas a short biasing time (<3 rain) leads to the deposition of good quality diamond films in our particular MW-CVD system.
3.2. Hydrogenplasma etch#lg of the deposit produced by sample biasing The chemical nature of the carbon precursor to CVD diamond formed during the biasing stage was studied through the effect of in situ hydrogen plasma exposure prior to growth. In these experiments, the Si(100) substrates were put through a three-step process: (i) bias pretreatment, (ii) exposure to a pure hydrogen plasma and (iii) deposition under normal conditions.
528
L Gou:man et al. / Diamond and Related Materials 6 (1997) 526-531
I I.
Deposition Time: (a)~1
i i
C~ *mat
~'
II.
Bias time; 7 rain. Etching time; 0 min.
Diamond (100) la - Type
]
60 min.
Bias Time:
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One can expect that if the carbon precursor, formed during the biasing stage, undergoes chemical etching in a hydrogen plasma atmosphere, then no diamond film should be formed at the end of the three-step deposition process. However, our AFM studies did not detect any difference in nucleation density as a result of exposure of the substrate to a pure hydrogen plasma. This effect was observed alter 7 min of bias pretreatment followed by 30min of hydrogen plasma exposure and l h of deposition. Fig. 2-I shows the Raman spectra of CVD films grown for I h, after 7 rain of bias pretreatment and followed by exposure to hydrogen plasma for different times, ranging from 0 to 30 min. A relatively prolonged bias time was chosen in order to obtain sufficient intensity of spectroscopic signals (Raman and AES) at the end of the biasing process. No visible effect of hydrogen plasma exposure is seen in the Raman spectra. The characteristic diamond peak at 1333 cm-1 was observed in all spectra, even after 30 min of hydrogen plasma exposure prior to film deposition. In Fig. 2-11 (b-eL high resolution C(KVV) Auger line shapes are presented as a function of the exposure time. The Auger spectrum taken for natural diamond is also shown in Fig. 2-11(a), as a comparison. The position of the C( KVV ) Auger line, and the line shape measured for the deposited films, are very similar to those of natural diamond, particularly after 20 and 30 min of hydrogen plasma exposure (Fig. 2-II (d) and (e)). It is
Fig. 2. I. Raman spectra of the different samples after 1 h of deposition under normal conditions following exposure of the films to hydrogen plasma for different times. II. High resolution C(KVV) electron excited Auger spectra taken after 1 h of deposition under normal conditions following exposure of the films to hydrogen plasma for different times.
important to stress that the Raman spectra of these films were also rather similar and displayed a significant contribution by an amorphous carbon phase. These results reflect the depth non-uniformity of the deposited films due to the different sampling depth of Raman and Auger spectroscopy. AES is a surface-sensitive technique, whereas Raman spectroscopy analyzes the bulk of the film. In order to gain insight into the nature of the carbon phase formed during sample biasing, films which underwent stages (i) and (ii) mentioned above (i.e., bias treatment and exposure to hydrogen plasma, without consequent CVD growth) were analyzed by Raman, AES and AFM. Fig. 3 shows the AFM pictures of Si substrates after different surface treatments: (a) untreated, (b) after 30 min of hydrogen plasma exposure, (c) after 7 min of the bias treatment and (d) after 7 min of the bias treatment followed by 30 min of hydrogen plasma
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Fig. 3. AFM pictures of Si substrate after diffeient surface treatments: untreated (a), after 30 min of hydrogen plasma exposure (b), after 7 min of the bias treatment (c) and after 7 min of the bias treatment followed by 30 min of hydrogen plasma exposure (d).
I. Gouzman et al. / Diamond and Related Materials 6 (1997) 526-531
exposure. Hydrogen plasma exposure, applied to virgin Si, resulted in an increase of the surface roughness (Fig. 3(b)). We attempted to grow diamond film on the substrate pretreated in the hydrogen plasma; however, no diamond particles were detected after 1 h of deposition (data not shown). Hence, the nucleation enhancement observed after 7 min of bias pretreatment and 30 min of hydrogen plasma exposure, followed by 1 h of CVD growth, is really a result of the stability under the etching conditions of the carbon precursor for diamond nucleation formed during the biasing stage. It was found that 7 min of bias treatment resulted in the appearance of large "hillocks" on the substrate surface (Fig. 3(c)). An increase in surface irregularity, concurrent with a decrease in the average height of the surface image, was observed after 30 min of hydrogen plasma exposure following 7 min of bias treatment (Fig. 3(d)). These changes may be associated with the roughening of the substrate surface by hydrogen plasma erosion and the partial etching of carbon and silicon atoms. In Fig. 4 the Raman spectra taken after 7 min of biasing pretreatment are shown as a function of hydrogen plasma exposure time. From these data it may be concluded that: (a) All spectra show the presence of two broad bands with apparent maxima at ~ 1333 and 1583 cm-1. These bands may be considered as D and G peaks of micro(or nano-) crystalline graphite, respectively. The large shift of the D peak (the D peak for microcrystalline
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omal
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Wavelength (cm -l) Fig. 4. Raman spectra of the different samples ariel" 7 min of bias following exposure of the films to hydrogen plasma for different times.
529
graphite is expected to be centered at 1350 cm -1) is possibly due to crystalline size effects. It may be suggested that very small diamond particles, formed during the biasing stage, contribute to the intensity of the Raman band at about 1333 cm-1. (b) The intensity of these lines does not change significantly after 10 or 20 min of hydrogen plasma exposure (Figs. 4(b) and (c)). This result, along with AFM studies, indicates a surprising stability of the carbon precursor deposited during sample biasing under exposure to hydrogen plasma. Hence, it is suggested that this precursor consists of nanocrystalline diamond particles, possibly embedded in a dense matrix of nondiamond carbon. This suggestion is supported by the results observed using selected area electron diffraction and X-ray diffraction, as well as EELS, in the works of McGinnis et al. [23] and Reinke et al. [24], respectively. Also, TEM examination of diamond films deposited using MW-BEN to promote nucleation revealed the presence of amorphous carbon at the silicon-thin film interface [2]. (c) An abrupt decrease in the intensity of the Raman lines was found in the spectra taken after 30 min of exposure to hydrogen plasma. However, a Raman spectrum taken after 1 h of deposition, following 7 min of sample bias and 30 min of hydrogen plasma exposure, does not differ significantly from those taken after shorter exposure times (see Fig. 2-l). The abrupt decrease in the Raman band intensity between 20 and 30 min of hydrogen plasma exposure may be associated with the relationship between the sampling depth of the Raman technique and the thickness of the deposited carbon layer. Thus, one can conclude that a very thin, perhaps non-continuous film of carbon precursor which remains on the Si substrate surlhce after exposure to hydrogen plasma for 30 min, is sufficient tbr nucleation enhancement. The surface composition of the substrates which underwent bias pretreatment followed by hydrogen plasma exposure (without the deposition stage), was assessed by AES. The Auger spectra of these samples show the presence of Si(LVV ), O(KLL) and C(KVV) Auger lines. High resolution C(KVV) and Si(LVV) Auger line shapes of substrates alter 7 rain of bias pretreatment, are presented as a function of hydrogen plasma exposure time in Fig. 5-1 and Fig. 5-I1, respectively. As a comparison, the C(KVV) and Si(LVV) Auger line shapes, measured for a reference silicon carbide surface, are shown in Fig. 5-1(a) and Fig. 5ll(a), respectively. The C(KVV) Auger line, measured for these samples, was observed at 270.5 eV. The line shapes are not characteristic of that measured for diamond. At the same time, the Si(LVV) Auger line shape, measured after 7 rain of the bias pretreatment, indicates the presence of silicon carbide on the substrate surface, as determined by the characteristic Si(LVV) line at
I. Gouzman et ai. / Diamoml ami Rehtted Materials 6 (1997) 526-531
530
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crystalline graphite, or silicon carbide. Therefore, it may be suggested that the precursor to diamond growth, deposited during the biasing process, consists of nanocrystalline diamond particles. (2) Under our experimental MW-BEN and subsequent deposition conditions it has been observed that a very short bias pretreatment time (1.5 min) is sufficient for nucleation of a high quality diamond film with a relatively high nucleation density (1 x 108 particles/cm2).
]
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ENERGY
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(eV)
Fig. 5. 1. High resolution C(KVV) electron excited Auger spectra taken alter 7 rain of bias following the exposure of the films to hydrogen plasma for different times. II. High resolution Si(LVV) electron excited Auger spectra taken after 7 rain of bias following exposure of the films to hydrogen plasma for different times.
87 eV (Fig. 5-11(b)). The intensity of this line decreases significantly after l0 min of hydrogen plasma exposure, concurrent with the appearance of features characteristic of silicon oxide at 62 and 78 eV [27] (Fig. 5-11(c)). After longer exposure times (20 and 30 min) no silicon carbide was detected on the substrate surface. From the observed results one can conclude that: (a) The C(KVV) Auger lines, measured after 0 and 10 min of exposure in hydrogen plasma (Fig. 5-1(b) and Fig. 5-1(c)), are composed of Auger electrons, originating from (i) carbon a~oms bonded to silicon in a silicon carbide configuration and (ii) carbon atoms bonded to each other. Unfortunately, it is difficult to assess the chemical configuration of these carbon atoms from the complex C(KVV) Auger line shape. (b) Only carbon atoms bonded to each other contribute to the C(KVV) Auger line, measured after 20 and 30 rain of hydrogen plasma exposure (Fig. 5-l(d) and Fig. 5-1(e)). It seems that these carbon atoms are those that serve as nucleation sites for subsequent diamcnd deposition and not SiC as the latter is etched by atomic hydrogen without a degradation effect on diamond growth. This result is strongly supported by the recent publication of Kulisch et al. [19].
4. Conclusions (1) We propose that hydrogen plasma exposure may be used as a chemical probe to determine the nature of the carbon precursor for diamond deposition, formed during the biasing process. In our initial studies it was found that this carbon phase was stable under the etching process, suggesting that it does not consist solely of a carbon network etchable by the hydrogen plasma, such as amorphous carbon (sp 2- or spa-bonded), micro-
Acknowledgement The authors gratefully acknowledge financial support of the Israeli Ministry of Science and Arts, project No. 1000 004 and the Technion Foundation for Research.
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[22] R. Stockel, K. Janischowsky, S. Rohmfeld, J. Ristein, M Hundhausen and L. Ley, J. Appl. Phys., 79 (1996) 768. [23] S.P. McGinnis, M.A. Kelly, S.B. Hagstr6m and R.L. Alvis, J. Appl. Phys., 79 (1996) 170. [24] P. Reinke, P. Ka.nia, P. Oelhafen and R. Guggenheim, Appl. Phys. Lett., 68 (1996) 22.
5? 1
[25] Z. Feng, M.A. Brewer, K. Komvopoulos, I.G. Brown and D.B. Bogy, J. Mater. Res., 10 (1995) 165. [26] A. Hoffman, A. Fayer, A. Laikhtman and R. Brener, J. Appl. Phys., 77 (1995) 3126. [27] O.S. Chia, C.M. Younes, P.G. Partridge, G.C. Allen, P.W. May and C.A. Rego, J. Mater Sci., 29 (1994) 6397.